Consume or be consumed: breaking down the structure of a food web By National Geographic Society, adapted by Newsela staff on 03.12.19 Word Count 1,005 Level 640L

Many organisms make up a food web. Animals like zebras are herbivores, or consumers that eat only plants. Lions are carnivores, or animals that eat other consumers.

Every living being is part of a food chain. Food and the animals that eat the food make up a food chain. For example, plants and grasses are food for zebras. Zebras are food for lions. Plants, zebras and lions make up a food chain.

There are many different food chains in an ecosystem. All together, the food chains in the ecosystem make a food web.

Trophic Levels

Organisms in food webs are grouped into categories. These categories are called trophic levels.

Producers

Organisms in the first trophic level are called producers. Plants are producers. Algae and some bacteria are also producers. Each producer makes its own food. Most producers use

This article is available at 5 reading levels at https://newsela.com. photosynthesis. This is a series of chemical reactions. Plants use these reactions to make energy from sunlight, carbon dioxide and water.

Consumers

The next trophic levels are animals that eat producers. These animals are called consumers.

The first level of consumers is made of herbivores. These animals are also called primary consumers. They eat plants, algae and other producers. Deer, mice and elephants are herbivores. They eat grasses, shrubs and trees. In the desert, a mouse is a primary consumer. It eats seeds and fruit. In the ocean, many fish and turtles are herbivores. They eat algae and seagrass.

Secondary consumers eat herbivores. In a desert, a secondary consumer may be a snake. It eats mice. In underwater kelp forests, sea otters are secondary consumers. They hunt sea urchins.

Animals in the next level are called tertiary consumers. They eat secondary consumers. In the desert, an owl or eagle may hunt snakes.

Top predators are also called apex predators. They eat other consumers. No other consumers eat them. Lions are apex predators on the grasslands of Africa. In the ocean, the great white shark is an apex predator. In the desert, bobcats and mountain lions are top predators.

Consumers can be carnivores or omnivores. Carnivores only eat meat. Omnivores eat both meat and plants.

Detritivores And Decomposers

Detritivores and decomposers make up the last part of food chains. Detritivores eat plants and animals that are not alive. For instance, vultures eat dead animals.

Some organisms, like fungi and bacteria, are decomposers. They turn decaying plants into soil. Decomposers allow food chains to start over. For example, grass makes its own energy through photosynthesis. A rabbit eats the grass. Then a fox eats the rabbit. When the fox dies, worms and fungi break down its body. The body returns to the soil. There, it leaves nutrients for plants to grow.

Biomass

Biomass is the energy in living organisms. Producers use the sun's energy to create biomass. The higher the trophic level, the lower the biomass. There is more energy in lower trophic levels than in higher ones.

There are always more producers than herbivores in a healthy food web. A healthy food web has many producers and many herbivores. It only has a few carnivores and omnivores.

This article is available at 5 reading levels at https://newsela.com. Every part of a food chain is connected to other food chains. When one part is in danger, others are also at risk. If plants are destroyed, herbivores don't have enough to eat. Their numbers go down. The number of plants can decrease because of drought or disease.

Humans can also destroy food chains by destroying habitats. People cut down forests. We use the lumber for buildings. We also pave over grasslands to build shopping malls or parking lots.

Bioaccumulation

Sometimes, pesticides can affect food chains. Pesticides get into the soil and water. Animals eats plants that are covered in pesticides. The pesticides stay in the animals' fat. When a carnivore eats that animal, it also eats the pesticides. This is called bioaccumulation.

Bioaccumulation happens in water ecosystems, too. Runoff from cities or farms can be polluted. Algae, bacteria and seagrass absorb the pollutants. Sea turtles and fish eat the seagrass. Then, sharks or tuna eat those fish. When people finally eat the tuna, that meal is full of pesticides.

In the 1940s and 1950s, bald eagles began disappearing. One major cause was a pesticide called DDT. The name DDT stands for dichloro-diphenyl-trichloroethane. It was used to kill insects that spread diseases. DDT builds up in soil and water. Worms, grasses, algae and fish ate organisms with DDT. Bald eagles ate the fish. They had high amounts of DDT in their bodies. They got it from their prey. These eagles started laying eggs with thin shells. These shells often broke before the baby birds hatched.

The U.S. government decided to ban DDT. Food webs have come back in most parts of the country. Bald eagle chicks are able to hatch.

Fast Facts:

Lost Energy

Higher trophic levels have less biomass. That is because most of an organism's biomass, or energy, is lost as heat or waste. A predator eats only the biomass that is left.

A Million To One

Marine food webs are usually larger than food webs on land. Scientists have measured how large. If a food web has a million producers, like algae and seagrass, it may only have 10,000 herbivores. This food web may only have 100 secondary consumers, like tuna. At the top of this massive food web, there is only one apex predator. This apex predator could be a human being.

Out For Blood

One of the first people to talk about food webs was Al-Jahiz. He was a scientist in Baghdad, Iraq. He lived in the early 800s, almost 1,200 years ago. Al-Jahiz wrote about mosquitoes preying on the blood of elephants and hippos. He understood that although mosquitoes preyed on other animals, they were prey, too. They were eaten by animals such as flies and small birds.

This article is available at 5 reading levels at https://newsela.com. Quiz

1 Why are fish, snakes, and lions all considered consumers?

(A) They eat other organisms to get energy.

(B) They all eat plants.

(C) They turn dead organisms into soil.

(D) Each one is consumed by some other organism.

2 Read the section "Biomass."

Which sentence from the section supports the idea that producers are needed for a healthy food chain?

(A) Biomass is the energy in living organisms.

(B) If plants are destroyed, herbivores don't have enough to eat.

(C) The number of plants can decrease because of drought or disease.

(D) We also pave over grasslands to build shopping malls or parking lots.

3 How are detritivores and decomposers important in a food chain?

(A) They turn dead animals and decaying plants into soil nutrients for plants to use again.

(B) They make sure pollutants do not bioaccumulate in the ecosystem.

(C) They are eaten by the producers.

(D) They balance the biomass when there are too many consumers.

4 Read the following paragraph from the section "A Million To One."

Marine food webs are usually larger than food webs on land. Scientists have measured how large. If a food web has a million producers, like algae and seagrass, it may only have 10,000 herbivores. This food web may only have 100 secondary consumers, like tuna. At the top of this massive food web, there is only one apex predator. This apex predator could be a human being.

What conclusion can the reader make from this paragraph?

(A) There are many more apex predators in the oceans than there are on land.

(B) Human beings are the only apex predator in water or on land.

(C) Algae and seagrass are less important than tuna and sharks.

(D) There are many more producers in the oceans than apex predators.

5 How do producers get their energy?

(A) by absorbing it from the soil

(B) from the primary consumers that eat them

(C) through a process called bioaccumulation

(D) through a series of chemical reactions called photosynthesis

This article is available at 5 reading levels at https://newsela.com. 6 Complete the sentence.

Decomposers cause ____.

(A) chemicals that build up in animals in the food chain

(B) chemical reactions that make energy from sunlight

(C) food chains to start over with nutrients in the soil

(D) food chains to have lower biomass near the top

7 How is the sun is the original source of energy for all animals.

(A) Animals absorb sunlight through their skin, providing them with energy.

(B) Through bioaccumulation, sunlight is passed through the food chain.

(C) Plants use sunlight to make their own food and animals eat plants or other animals.

(D) When decomposers break down plants they make the sun's energy available for animals to use.

8 How did bioaccumulation hurt bald eagles?

(A) It caused pesticides to kill the bugs and creatures that the eagles counted on for food.

(B) It caused pesticides to build up in the food chain that made the eagles' egg shells too thin.

(C) It caused the forest habitats where the eagles lived to be cut down for buildings.

(D) It caused the forest habitats to lose their energy by killing their habitat's producers.

This article is available at 5 reading levels at https://newsela.com.